Study Doubles Number of Potential Earth-Like Planets

Study Doubles Number of Potential Earth-Like Planetary Systems

The standard model for the formation of rocky planets like Earth appears to be on solid ground even when applied to systems in which two stars orbit each other, according to a new study.

If true, the study also effectively increases the potential number of Earth-like planets in the Universe.

The new work, led by Stephen J. Kortenkamp at the University of Maryland, is discussed in the Aug. 10 issue of the journal Science. It bolsters the hope that potentially habitable planets might form around other stars.

The work involved a computer model that showed how the gravity from a large object, such as a second star or a large planet, tugs at young asteroids in a nascent solar system and causes collisions that can create rocky, Earth-sized planets.

Based on models of how our solar system formed, researchers have long assumed that most regular, single stars have the potential to harbor planets similar to Earth in size and orbital characteristics. There are billions and billions of such stars just in our own Milky Way Galaxy.

But roughly half of what appear to be single stars are actually binary star systems.

"Since binary stars are so common, this result could roughly double the number of stars that might support Earth-like planets," said Alan Boss, an expert on planet formation at the Carnegie Institution of Washington.

Challenges to the standard model

For decades, scientists have had a pretty good handle on how planets form. There are some holes in the main theory, such as exactly how Jupiter got going on the growth spurt that made it so big. Even more uncertain is how Uranus and Neptune came to exist at all. But by-and-large astronomers agree on how Earth and the other three rocky planets of the inner solar system evolved.

Yet in the past six years, researchers have learned that there are more planets outside our solar system than there are in it.

Most of these extrasolar planets, or exoplanets, exhibit one glaringly different characteristic from anything scientists had ever seen before 1995: Many are much larger than Jupiter yet they orbit closer to their host stars than Mercury does to our Sun. Further, more than a fourth of the more than 50 exoplanets found so far orbit around a star in a binary star system — a pair of stars that also orbit one another.

The conventional view of planet formation has suddenly been exposed to intense scrutiny.

This so-called standard model holds that after our Sun formed, uncountable dust grains and pebble-sized rocks collided and coalesced over tens of thousands of years, forming trillions of asteroid-like objects, rocks called planetesimals that were as big as cities.

For millions of years thereafter, a period called "runaway growth" ensued during which these giant rocks collided and merged to form just a few dozen huge planetary embryos. Eventually these embryos joined forces to create the inner four "terrestrial" planets — Mercury, Venus, Earth and Mars.

But scientists are not sure how Jupiter and other gas giants, including those around other stars, form. Some suggest that the same runaway growth causes a planetesimal 10 times the size of Earth to develop first, then it begins to gather gas in a process spanning some 10 million years.

This would mean Jupiter has a rocky core, but no one has proved that.

Other researchers think Jupiter might have formed by what is known as a gravitational collapse, when a knot of dense material orbiting the nascent Sun collapsed under its own weight and quickly formed a full-sized Jupiter. The Sun and other stars form in such a collapse, but researchers don’t know the lower limit of mass that can support such an event.

Researchers also wonder whether the runaway growth predicted by the standard model could produce Earth-like planets in binary star systems.

Adding a gas giant to the model

Kortenkamp worked with George Wetherill and Satoshi Inaba at the Carnegie Institution of Washington to try an answer some of these questions. They employed a new hybrid technique that uses two computer programs that many researchers use to theorize about planet formation. The software simulates gravitational interactions between small and large objects and predicts what will happen over time based on whatever criteria the researchers start with.

Kortenkamp and his colleagues identified a new type of runaway growth that allows the standard model of Earth’s formation to be applied in more complicated systems. The new model depends on the presence of another large object besides the main star — either another star or a gas giant planet. The gravity from this large object tugs at planetesimals and encourages collisions that might eventually yield a Moon-sized object.

The team started with a model of our Sun, Jupiter and Saturn, along with a large population of small rocks. They then let virtual gravity run for about a million years, a process that took several months in real time, and watched what happened.

What happened was that planetary embryos, the precursors of terrestrial planets did in fact form, even under the gravitational influence of the giant gas planets that were already there. So regardless of how Jupiter formed, the standard model that explains Earth’s formation does in fact work, Kortenkamp said in a telephone interview.

"We aren’t going to have to reinvent it," he said.

Boss, of the Carnegie Institution, agreed. He collaborates with some of the researchers on the team that did the new study but was not directly involved in it.

"There really is no valid alternative to the standard model for forming terrestrial planets," Boss told "So the fact that this study strongly reinforces the standard model means that we may rest secure in our belief that we understand the basics of terrestrial planet formation, though the exact details always change a bit."